1. Field of the Invention
The present invention relates to an electrode assembly and a secondary battery having the same, and more particularly, to a secondary battery that can maximize battery safety without degrading battery performance.
2. Description of the Related Art
In recent years, the rapid development of small and lightweight portable electronic devices has generated an increasing need for high-capacity, small-sized batteries. In particular, lithium ion secondary batteries can provide an operating voltage of at least about 3.6 V, which is about 3 times higher than nickel-cadmium batteries or nickel-hydrogen batteries widely used in portable electronic devices, and they have a higher energy density per unit weight than nickel-cadmium batteries or nickel-hydrogen batteries. For these reasons, research into lithium ion secondary batteries has rapidly progressed.
In a lithium ion secondary battery, electrical energy is generated due to oxidation and reduction reactions, which occur when lithium ions are intercalated/deintercalated at positive and negative electrodes. Fabrication of the lithium ion secondary battery involves using materials capable of reversibly intercalating/deintercalating lithium ions as active materials of the positive and negative electrodes and filling an organic electrolyte or polymer electrolyte between the positive and negative electrodes.
The lithium ion secondary battery includes an electrode assembly in which a negative electrode plate and a positive electrode plate with a separator interposed therebetween are wound in the form of, for example, a jelly-roll, a can for containing the electrode assembly and an electrolyte, and a cap assembly mounted on the can.
Conventionally, a single or multiple polyolefin-based micro-porous polymer layer formed of at least one of polypropylene (PP) and polyethylene (PE) was used as the separator. However, since the polyolefin-based micro-porous polymer layer serving as the separator has a sheet or film shape, when heat is generated due to internal shorting or overcharge, pores may be clogged and the film-type separator may shrink.
Accordingly, when the film-type separator shrinks due to heat generated in the lithium ion secondary battery, portions of the positive and negative electrodes, which are not separated by the shrunk separator, are brought into contact with each other, thereby causing ignition, bursting, or explosion.
In order to make up for these weak points in the film-type separator, a considerable amount of research has focused on forming a ceramic separator using a porous layer formed of a combination of a binder and a ceramic material, such as silica (SiO2), alumina (Al2O3), zirconium oxide (ZrO2), or titanium oxide (TiO2).
In this case, the ceramic separator may make up for the fusion and shrinkage of a film-type polyolefin separator at a high temperature of about 120° C. or higher. As a result, there is a growing tendency to use both a conventional film-type separator and a ceramic separator.
However, when both the film-type separator and the ceramic separator are used, battery safety is improved, but the film-type separator must be as thin as the ceramic separator in order to obtain the same cell capacity as a conventional secondary battery.
In detail, when a ceramic layer has a predetermined thickness or less, the safety of the ceramic layer is not improved, while when the ceramic layer exceeds the predetermined thickness, battery performance is deteriorated. In addition, when the film-type separator has less than a predetermined thickness, the thermal characteristics of the secondary battery are deteriorated, and when the film-type separator is too thick, battery performance is deteriorated.
Accordingly, it is necessary to optimally design a lithium ion secondary battery in consideration of compatibility between the film-type separator and the ceramic separator so as to maximize battery safety without degrading battery performance.